This invention relates to the field of quantum well optical nonlinear techniques and to the use of spatially periodic electric field bias and exciton confinement to enhance third order nonlinear optical effects.
Nonlinear optical waveguides are an essential component in a number of proposed optical and electronic-optical systems which are of interest to the U. S. military. Presently available optical waveguide materials, materials such as lithium niobate, are inadequate for use in many of these systems as a result of several limiting factors. Such limitations include fabrication difficulties, slow response times, low power handling capability, and especially relatively small third-order nonlinear optical coefficients. Larger nonlinear optical coefficients along with fast sub-nanosecond response times are however, available in combined group III and group V periodic table semiconductor materials under appropriate operating conditions.
The nonlinearity coefficient available from such materials is particularly striking in that for example, third order nonlinear coefficients (.chi..sup.(3)) in the range of 0.24.times.10.sup.-0 esu are available for bulk gallium arsenide, however, coefficients as large as 10,000,000.times.10.sup.-9 esu are available from gallium arsenide/aluminum gallium arsenide multiple quantum well structures. These larger nonlinear coefficients are achieved by band gap engineering in which the principal physical mechanism employed is the confinement of semiconductor electron-hole pairs, i.e., confinement of excitons in one dimension. Even these large nonlinear coefficients can be greatly enhanced by the additional confinement of excitons in two or even three dimensions. Such additional confinement arrangements have been achieved at least experimentally through etching of a confining structure as is disclosed in references 8 and 9 herein, by pattern ion implantation and annealing, as is disclosed in references 10 and 6 herein, and also by strain-induced lateral confinement, as is disclosed in reference 5 herein.
The concept of creating a periodic electric potential in the plane of a semiconductor multiple quantum well structure for the purpose of increasing electron mobility has been considered in reference 1 herein. The use of this multiple quantum well structure for enhancing electron mobility as described in this reference, however, is significantly different from the concept of the present invention and perhaps more importantly, the physical mechanism being utilized in reference 1 differs sharply from that of the present invention.
The issued patent art also discloses several examples of multiple quantum well structures applied to the achievement of nonlinear optical effects. Included in this patent art is the U.S. patent of M. M. Fejer et al., U.S. Pat. No. 4,880,297, which is concerned with a quantum well optical electric field biased nonlinear method and apparatus. The Fejer et al invention employs gallium arsenide and aluminum gallium arsenide optical structures disposed in optical waveguide elements and controlled by spatially periodic electrode members to introduce quadratic nonlinear susceptibility into the optical propagation characteristics of a quantum well layer structure.
It is notable however, that the Fejer et al quantum well structure is disclosed to produce quadratic nonlinear optical susceptibility, that is, an effective asymmetric .chi..sup.(2) effect as opposed to an enhancement of the .chi..sup.(3) effect as in the present invention. It is also notable that the Fejer et al disclosure is concerned with wavelength mixing utilization of the quantum well structure as opposed to uses in the optical switching and modulation art for the present invention. It is also notable that the Fejer et al apparatus employs a spatial period separation of the electrode elements and the achieved electric field--a spacing on the order of the coherence length of the desired three-wave interaction process (see column 4, lines 1-5). This spacing is opposed to a spacing period near the atomic Bohr radius in the present invention. The Bohr radius dimension is normally smaller by at least a few orders of magnitude than the coherence length disclosed in the Fejer et al apparatus.
The art of interest also includes the patents of U. Efron et al, and D. W. Langer et al., U.S. Pat. No 4,828,368 and U.S. Pat. NO. 4,923,264 which are concerned with near bandgap radiation modulation spatial light modulators and a resonance coupled optical coupler with semiconductor waveguide layer comprising a multi-quantum-well structure, respectively. The Efron et al. and Langer et al. patents are each concerned with multiple quantum well arrangements wherein uniform electric fields are applied to the quantum well structure, as opposed to the spatially periodic fields of the present invention. In addition, the Efron et al device uses the electric field to shift the spectral location of an absorption phenomenon according to the Franz-Keldysh effect. The Langer et al patent relies on a resonant coupling mechanism between two waveguide layers with the coupling being dependent upon equal indices of refraction in the two layers and with the index of one layer being determined by quantum well effects and being responsive to an applied electric field.
Additional information which may be of background interest with respect to the present invention is included in the list of references appended to the specification and in the citation of prior art included in a disclosure statement which accompanies the patent application. The concepts disclosed in these appendix and disclosure statement listed publications are believed to be principally of background and less relevant interest in comparison with the three patent references listed above.